Patients with clinical evidence of cerebral vascular insufficiency frequently relate that a particular head position may precipitate their symptoms. Usually, extension of the head, as in looking upward, or lateral rotation, as in looking over the shoulder, is mentioned. In a previous study1 we found that many such patients tolerated compression of a carotid artery when their heads were face forward but quickly developed signs and symptoms of insufficiency when carotid compression was repeated after the head had been turned to one side. In the past, those2,3 who have recognized this influence which head position can have upon response to carotid compression have usually attributed it to stimulation of a carotid sinus reflex mechanism or to local anatomical factors which permit more effective compression of the sinus when the head is rotated. In this paper we suggest that a more important factor may be change in blood flow